This invention relates to the monitoring of yarn whilst it is being processed, and in particular to the monitoring of the regularity of a multifilament yarn in order to detect broken filaments or protruding filament loops.
During the processing of a multifilament yarn it is possible that filaments become broken or filament loops are formed. Such broken filaments or loops tend to protrude from the surface of the yarn, thereby reducing the efficiency of the subsequent processes performed on the yarn, such as warping, knitting, weaving. The broken filaments or loops subsequently protrude from the surface of a fabric made from that yarn, which reduces the quality and appearance of the fabric. It is therefore desirable to monitor the occurrence of protruding broken filaments or loops during the processing of the yarn so that unacceptable yarn may be rejected instead of being made up into an unsatisfactory fabric. Such monitoring also serves to draw attention to factors during the processing of the yarn that affect the incidence of broken filaments or loops, so that attempts can be made to change the processing parameters or modify the apparatus in order to reduce the number of broken filaments or loops and produce an ideal yarn. By ideal yarn is intended to mean a yarn without any broken filaments or filament loops protruding to an unacceptable level from the surface of the yarn.
Two principal methods of monitoring the processing of multifilament yarns for broken filaments or protruding filament loops are currently in operation. In the first method, a light beam is directed adjacent the surface of the moving yarn as it is being processed. If a broken filament or filament loop protrudes from the surface of the yarn it is likely to intrude briefly into the light beam and the amount of light received by a receiving device is temporarily reduced. In the second method, a light beam is directed at the moving yarn during processing, the light passing either side of the yarn being received by a receiving device. If a broken filament or filament loop protrudes from the surface of the yarn, the effective cross-sectional area of the yarn in the light beam is briefly increased and again the amount of light received by a receiving device is temporarily reduced. With both methods, the number of such reductions in the amount of light received in a given time, corresponding with the passage of a given length of yarn, is recorded. This indicates the number of broken filaments or loops per unit length of yarn.
In the case of the first method, detection of a broken filament or filament loop is achieved only if that filament protrudes from the surface of the yarn in a plane such that it intrudes into the light beam. Also, the clearance between the light beam and the surface of the yarn is critical, and has to be altered for different yarns since the regularity of the xe2x80x98cylindrical surfacexe2x80x99 presented by the yarn is dependent on the type of yarn, the twist level, and/or the degree of texturing, interlacing and the like. In an attempt to overcome the first problem, the light beam may be directed adjacent the yarn as it passes around a roller, so that the broken filaments or filament loops tend to be directed outwardly into the light beam. However any broken filaments or filament loops that become trapped between the yarn and the roller are not detected. This problem occurs to a far lesser extent with the second method. However, a comparable second problem exists, in that it is difficult to set the tolerance or xe2x80x98threshold levelxe2x80x99 for the amount of light being received by the receiving device before a broken filament or filament loop is recorded due to the above mentioned non-regularity of the xe2x80x98cylindrical surfacexe2x80x99 presented by the yarn. The setting may even have to be adjusted from position to position on a single yarn processing machine or even with time on a single position, due to variation in local contamination, optical qualities of the light emitting and receiving devices and other local factors.
With either method, the detection of broken filaments in practical thread-lines is often made more difficult due to unavoidable disturbances to the thread-line. These are caused, for example, by longitudinal xe2x80x9cpulsingxe2x80x9d of the yarn before and after an interlace jet, or by transverse vibration of the yarn as it is delivered at high speed through the processing apparatus.
It is an object of the present invention to provide, whilst monitoring the protrusions, i.e. protruding broken filaments or loops, in a multifilament yarn during the processing thereof, a method of adjustment of the threshold level in order to overcome to a significant extent the problems associated with the second method as currently used. In addition, the invention seeks to distinguish the effects of the above-described disturbances on the shadowed yarn signal from those of broken filaments.
The invention provides a method of monitoring the processing of a multifilament yarn, in which a light beam is directed from a light emitting device to a light receiving device, a multifilament yarn is passed through the light beam and the amount of light received by the light receiving device is measured, comprising measuring the amount of light received at predetermined time intervals, producing a frequency distribution from the measured amounts of light over a predetermined time period, calculating from the frequency distribution a threshold level representative of an ideal yarn, and recording the number of measurements that fall outside that threshold level to indicate the number of protrusions in the yarn.
The predetermined intervals may be such that measurements are recorded at between 10000 and 50000 times per second depending on process speed, e.g. substantially 25000 times per second. The predetermined time period may be between 10 and 1000 milliseconds, preferably substantially 100 milliseconds.
The calculation may comprise calculating a normal value, which is that measurement within which a predetermined number of the measurements fall. The predetermined number may be between 95% and 100%, and may be 99%. When discontinuous samples of signal are used to form the distribution, it may be necessary for the method to comprise recording the normal values of a plurality of distributions and taking the mean or minimum value of such distributions. The calculation may also include adjusting the normal value by a sensitivity factor to determine the threshold level. The sensitivity factor may be between 1% and 50% of the normal value, depending on the size of the broken filaments or loops to be recorded.
The method may also include differentiating, by analogue or digital means, the measured amounts of light with respect to time. The differentiation may include determining the first derivative of the measured amounts of light, from which the frequency distribution is produced.
The invention also provides apparatus for monitoring the processing of a multifilament yarn, comprising a light emitting device operable to direct a light beam to a light receiving device operable to measure the amount of light received, further comprising a measuring and computing device operable to:
measure the amount of light received at predetermined intervals;
produce over a predetermined time period a frequency distribution of the measured amounts of light;
calculate from the frequency distribution a threshold level representative of an ideal yarn, and;
record the number of measurements that fall outside the threshold level to indicate the number of protrusions in the yarn.
The measuring and computing device may be operable to measure the amount of light received at between 10000 and 50000 times per second depending on process speed, e.g. substantially 25000 times per second. The measuring and computing device may be operable to produce a frequency distribution of the measured amounts of light over a time period of between 10 and 1000 milliseconds, preferably substantially 100 milliseconds.
The measuring and computing device may be operable to calculate a normal value, which is that measurement within which a predetermined number of the measurements fall. The predetermined number may be between 95% and 100%, and may be 99%. The measuring and computing device may record the xe2x80x9cnormal valuesxe2x80x9d of several distributions and take the mean or minimum values to get a true representation of the complete signal. The measuring and computing device may also be operable to adjust the normal value by a sensitivity factor to determine the threshold level. The sensitivity factor may be between 1% and 50% of the basic measurements.
The measuring and computing device may be operable to determine the first derivative of the measured amounts of light, from which the frequency distribution is produced.